1. Field of the Invention
The present invention relates to disk-shaped magnetic recording media, essentially consisting of a flexible substrate, a thin ferromagnetic metal film applied to one or both sides and a protective or antifriction layer present on this metal film and essentially consisting of carbon, a process for the production of these recording media and information stores which contain these recording media.
Disk-shaped magnetic recording media which have a magnetic layer, consisting of a magnetic powder material and an organic binder, on a flexible substrate are known in particular as floppy disks and are widely used. To increase the information density, it has often been proposed that the pigment/binder magnetic layer be replaced by such a layer consisting of ferromagnetic material in the form of a thin metal film. As a result of the increase in coercive force made possible by a thin metal film on the one hand and, on the other hand, the reduction of the layer thickness to less than 1,000 nm, made possible by the greater magnetization of the ferromagnetic metals or alloys used, a substantially higher storage density of the recording medium can be achieved. These thin metal films are preferably produced by PVD methods, such as cathode sputtering or vapor deposition. However, these thin metal films must be protected both against corrosion and against the mechanical stress due to the head. When selecting such protective layers, however, it is necessary to ensure that neither application of the protective layer nor the protective layer itself has any adverse effect on the magnetic layer.
2. Description of the Related Art
Many suggestions have been made for solving these problems. For example, U.S. Pat. No. 3,767,369 describes the application of a rhodium protective layer for improving the hardness and the anti-friction properties; it is necessary to apply a tin/nickel intermediate layer to improve the adhesion of the rhodium to the magnetic layer, which is too low. This process does not provide the protective layer properties now required, nor is the application of the said layer simple and without problems. Where the metallic magnetic layer contains cobalt, it has been proposed to heat this magnetic layer in the air at a predetermined humidity and hence to oxidize the surface (U.S. Pat. Nos. 3,353,166 and 4,029,541). However, such a process has particular disadvantages. Thus, the heating process required to produce the stated protective layers may influence not only magnetic properties of the recording layer itself but also conventional lower or intermediate layers in such a way that these in turn adversely affect the properties of the magnetic layers.
Furthermore, the application of liquid oligomers, for example of perfluoropolyethers, to the magnetic layer to be protected, by immersion or spin-coating, is disclosed in, for example, U.S. Pat. No. 3,778,308. However, the disadvantage of this process is that the lubricant migrates increasingly in an outward direction, for example under the influence of the centrifugal force and of the air flow during operation of the recording media, so that, with the decrease in lubrication, the wear and the friction increase very sharply and mutually influence one another.
In other processes, various protective layers are applied under reduced pressure, generally by sputtering, for example, according to U.S. Pat. No. 4,277,540, layers of gold, tantalum, niobium, platinum, chromium, tungsten and rhodium and the nitrides or carbides of silicon, of zirconium, of hafnium and of titanium and, according to U.S. Pat. No. 4,268,369, layers of silica. Furthermore, East German Patent 109,101 describes protective layers consisting of carbon layers grown under reduced pressure, for magnetic stores having a thin metal film. Special embodiments of such protective carbon layers are described in IEEE Trans. Mag., Vol. MAG-22, No. 5, September 1986, pages 999-1001 and, inter alia, also in WO 88/05953 or EP-A 273330. With these modified protective layers based on carbon, the abrasion properties of recording media having thin metal films can be improved so that they reach a service life of up to 11 hours, ie. up to 0.2 million revolutions, at a speed of 300 rpm. However, the specifications require the service life of disk-shaped recording media to be not less than 560 hours or 10 million revolutions, the head being continuously in contact with the recording track.